1. Field of the Invention
The present invention relates to an apparatus and a method for detecting an out of phase (loss of synchronism) occurring in power systems according to the power, current, and the like measured at a tie point between the power systems and, more specifically, to an apparatus for stabilizing power systems by detecting or predicting an out of phase occurring in the power systems and paralleling off a tie point between the power systems. The present invention is intended to detect and predict the first swing instability to the multi-swing instability.
2. Description of the Related Art
Oscillation of generator output power occurs in a power system because of various factors such as variations in load and a system accident. Small oscillation of generator output power occurs even when the power system is in a normal state. If, however, the oscillation of generator output power increases, it causes an out of phase condition finally, with the result that the entire power system is likely to collapse if the out of phase condition is not removed.
Conventionally, the oscillation of general output power respect to an electric parameter in the power system has been detected by various methods in order to predict or detect an out of phase condition appearing in the power system. For example, Jpn. Pat. Appln. KOKOKU Publication No. 4-52051 discloses an out of phase prediction apparatus. In this apparatus, a bus voltage and a line current are sampled at a constant cycle in any one of electric-power stations, substation, etc., connected together through transmission lines. A bus voltage and a current vector are detected from the sampled bus voltage and line current, and a bus voltage vector of another station is determined based on the detected value and vector. Then differences in phase angle among the stations are calculated from the real and imaginary parts of the bus voltage vector, and a difference in voltage phase angle is predicted from the calculated differences in phase angle, thereby predicting that an out of phase will occur in the power system based on variations in the difference in voltage phase angle.
Jpn. Pat. Appln. KOKOKU Publication No. 4-65618 teaches an out of phase detection apparatus. In this apparatus, the current and voltage of a power system are sampled at a predetermined cycle, and electric power is obtained from the sampled current and voltage. The out of phase of the power system is then detected from variations in the electric power with time. Moreover, instantaneous power at the present time is obtained from the sampled current and voltage, and it is determined from variations in the instantaneous power with time whether the oscillation of out of phase increases or decreases.
According to the conventional methods described above, a threshold value suitable for determining an out of phase is preset by simulation, and it is determined whether the out of phase occurs or not, according to whether a detected or predicted phase angle exceeds the preset threshold value.
Since, however, the configuration of the power system is changed frequently, the suitable threshold value changes, and the threshold value has to be reset accordingly.